1. Field of the Invention
The subject invention relates generally to the communications arts and, more particularly, to an improved phase lock loop for use in receiving signals of widely varying level and subject to Doppler shift.
2. Description of Related Art
Space flight requirements have inspired intensive application of phase lock methods. As reported in Gardner, Phase Lock Techniques, 2d Ed., Willey & Sons (1979), space use of phase lock begain with the launching of the first American artificial satellites. These vehicles carried low-power (10 mW) CW transmitters and the received signals were correspondingly weak. Because of Doppler shift and drift of the transmitting oscillator, there was considerable uncertainty about the exact frequency of the received signal. At the 108 MHz frequency originally used, the Doppler shift could range over a .+-.3-kHz interval.
With an ordinary, fixed-tuned receiver, bandwidth would therefore have to be at least 6 kHz, if not more. However, the signal itself occupied a very narrow spectrum and could be contained in a bandwidth of approximately 6 Hz.
Since noise power in the receiver is directly proportional to bandwidth, a noise penalty of 1000 times (30 dB) would have had to have been accepted if then-conventional techniques were used. The noise penalty has become even more severe. For example, with the movement of transmission frequencies to S-band, the Doppler shift range increased to approximately .+-.75 kHz while receivers with bandwidths as small as 3 Hz have been achieved. The noise penalty in such case is about 47 dB.
Noise can be rejected by a narrow band filter, but if the frequency of the filter is fixed, the signal may not be within the passband. For a narrow bandwidth filter to be usable it must be capable of tracking the signal. A phase locked loop is capable of providing both the narrow bandwidth and the tracking that are needed. Moreover, extremely narrow bandwidths are conveniently obtained with phase locked loops. Hence, narrow band, phase locked, tracking receivers have come into use to avoid severe noise penalties and to acquire drifting signals.
A phase lock loop (PLL) contains three basic components: a phase detector, a loop filter, and a voltage-controlled oscillator (VCO), whose frequency is controlled by an external voltage.
The phase detector compares the phase of a periodic input signal against the phase of the VCO. The difference voltage output of the phase detector is a measure of the phase difference between its two inputs. The difference voltage is then filtered by the loop filter and applied to the VCO. Application of the control voltage to the VCO changes the frequency in a direction that reduces the phase difference between the input signal and the local oscillator.
The most well-known phase lock loop is the second order loop, so-called because of the second order transfer function provided by its loop filter. A well-known second order loop filter configuration employs an operational amplifier having an input resistance and a feedback loop containing a second resistance in series with a capacitance.
A second variety of phase lock loop, the Haggai loop, employs a loop filter having a constant phase network in the operational amplifier feedback loop. In this design, the loop filter provides a nearly constant phase margin, and hence nearly constant per unit damping over a wide range of open loop gains. For low carrier to noise density ratios C/N, the carrier is buried in noise and is of unknown amplitude prior to PLL acquisition. Since the open loop gain of the PLL is proportional to carrier level, the PLL acquisition loop dynamics will change substantially with the unknown carrier level. The important advantage of the Haggai loop PLL over other designs such as the second order loop is that the per unit damping of the closed loop dynamics is nearly constant over a wide range of unknown input carrier levels. This feature greatly enhances the performance of the Haggai loop in acquiring an unknown Doppler shifted signal of widely varying power. At low frequencies, the Haggai loop reverts to a second order loop so that the steady state phase error is zero.
While the Haggai loop has made a significant contribution to PLL technology, its use in certain applications has been found to have some disadvantages. For example, the Haggai loop has exhibited a probability of loss of lock on the order of 30%, in switching from an acquisition mode to a narrow band track mode. This phenomenon can be explained heuristically by considering that at the instant of switching from a wide acquisition bandwidth to a narrow tracking bandwidth the frequency error may easily exceed the pull-in bandwidth of the tracking loop. The Haggai loop has further exhibited a propensity to false lock at high signal levels.
It would therefore be an advancement in the art to provide a receiver using PLL technology which includes the advantages of the Haggai loop but which does not lose lock when switching from the acquisition mode to the track mode and which does not false lock at high signal levels.